The present invention relates generally to a carrier recovery circuit for an offset QPSK (quadrature phase shift keying) demodulator.
Offset QPSK is a form of QPSK in which the digits in the quadrature channels ahve a relative delay in their transitions. If the serial input data have symbol duration T, then the I and Q data will each have symbol duration 2TT. The relative delay between channels is T. In conventional QPSK the transitions are coincident. The purpose of this delay is to restrict the carrier phase transitions from having 180.degree. phase transitions. When filtered, the offset QPSK will have less envelope fluctuation compared with QPSK. In the unfiltered case, the introduction of a delay has no performance effect and offset QPSK has the same error rate as does conventional QPSK. As is well known, signals are transmitted in the form of a succession of short-duration bursts in satellite communications system for minimizing satellite's power consumption and for enabling time division multiple access. In order to reduce demodulator signal acquisition time, bursts are structured so that each starts with a preamble containing bits that accentuate carrier clock frequency spectra. The preamble for each of the quadrature channels begins with a carrier recovery field which is followed by a clock recovery field. Specifically, the carrier recovery fields of both of the quadrature channels contain a succession of all binary 1's. Whereas the clock recovery field of in-phase channel contains a succession of all binary 1's as in the carrier recovery field and the clock recovery field of the quadrature channel contains a succession of alternating binary 1's and binary 0's. Since demodulators of satellite transmission systems operate at low carrier-to-noise (C/N) ratios and since the prable is of a short duration for channel utilizlation efficiency, it can occur that demodulators fail to successfully recover a carrier at the end of the carrier recovery field and hence the recovered carrier still contains a phase error at the beginning of the clock recovery field. This implies that carrier recovery action should continue after entering the clock recovery field. However, with conventtional carrier recovery circuits for offset QPSK demodulation, clock timing is not successfully recovered at the beginning of the clock recovery field. Even though a delay has been introduced in a Costas loop with a view toward coinciding phase transitions between in-phase and quadrature channels, it is impossible to continue to the carrier recovery process in the clock recovery field.